This invention relates to a gas-insulated switchgear apparatus, and more particularly to improvements of its connecting section which electrically connect the switchgear main section to three-phase AC power transmission lines by means of bushings.
In modern power generating stations, power switching stations and power substations, gas-insulated switchgears are widely used as switching equipment since the area required for the installation of the station can be reduced and the maintenance of the station components can be facilitated.
In the switchgear apparatus of this kind, a suitable gas such as SF.sub.6 gas having an excellent electrical insulating property is contained within a sealed vessel or enclosure of metal which is maintained at the earth potential, and high-potential loading conductors are disposed within this sealed enclosure in a relation electrically insulated from the enclosure by an electrical insulator. Due to the fact that the electrically insulating gas such as SF.sub.6 gas is contained within the sealed enclosure of the gas-insulated switchgear apparatus for the purpose of electrical insulation of the high-potential loading conductors from ground, the distance required for the electrical insulation between the conductors and ground, i.e. ground insulation distance, can be greatly shortened compared with that in a conventional air-insulated switchgear apparatus employed hitherto, and the distance required for the electrical insulation between different phases, i.e. interphase insulation distance, can also be greatly shortened. Thus, the overall size or volume of the switchgear apparatus can be greatly reduced.
However, because of the fact that the main conductors of the gas-insulated switchgear are connected to the three-phase AC power transmission lines through bushings, these bushings must be spaced apart from each other by distance enough to electrically insulate the bushings from each other in open air. Each bushing includes a porcelain insulator and a central conductor. The high-potential loading conductors in the gas-insulated switchgear apparatus are connected to these central conductors of the respective bushings to terminate in individual external terminals. Therefore, the open-air insulation distance must also be taken into consideration for ensuring electrical insulation between these external terminals exposed in open air. Because of the fact that this open-air insulation distance required for the electrical insulation between the exposed external terminals is far larger than the insulation distance required within the gas insulation circumstance, this results in the loss of the merit that the insulation distance can be shortened in other conductor portions than the portions insulated by the bushings. As a consequence, the size or volume of the gas-insulated switchgear apparatus is determined primarily by the insulation distance required for electrical insulation between the bushings.
This problem will be further discussed. It is the recent tendency to operate AC power transmission lines at higher voltages to meet the growing demand for electric power, and it is also the recent tendency to increase the number of paralle power transmission lines through main route to enhance the reliability of power supply therethrough. For example, there is a project for erection of a substation employing 550-kV gas-insulated switchgear arrangement associated with six parallel power transmission lines. Such gas-insulated switchgear arrangement includes generally line feeder switching units connected to the power transmission lines, bank feeder switching units connected to transformers, bus section switching units, bus tie switching units, main bus units and connection conductors for connection between the individual units. The main bus units are arranged to extend in a direction parallel with the direction required for maintaining a sufficient insulation distance between the bushings. Therefore, an increase in the distance between the bushings or an increase in the number of the lines results in a corresponding increase in the axial length of the main bus units, and the proportion of the cost of the main bus units in the overall cost of the gas-insulated switchgear arrangement is inevitably increased resulting in a very expensive substation. For the above reasons, it is strongly demanded to ensure the required open-air insulation distance between the bushings without increasing the axial length of the main bus units.